1. Field of the Invention
This invention relates to an electrolyte for an electrolytic capacitor, particularly a high voltage electrolytic capacitor, which requires enhanced reliability.
2. Background Art
Compact, high voltage capacitors are utilized as energy storage reservoirs in many applications, including implantable medical devices. These capacitors are required to have a high energy density since it is desirable to minimize the overall size of the implanted device. This is particularly true of an Implantable Cardioverter Defibrillator (ICD), also referred to as an implantable defibrillator, since the high voltage capacitors used to deliver the defibrillation pulse can occupy as much as one third of the ICD volume.
Implantable Cardioverter Defibrillators typically use two electrolytic capacitors in series to achieve the desired high voltage for shock delivery. For example, an implantable cardioverter defibrillator may utilize two 350 to 400 volt electrolytic capacitors in series to achieve a voltage of 700 to 800 volts.
A strategy for increasing energy density in the capacitor, and thus reducing its size, is to minimize the volume taken up by the paper and cathode foil and maximize the number of anodes. This may be achieved by using a multi-anode stack configuration. A multiple anode stack configuration requires fewer cathodes and paper spacers than a single anode configuration and thus reduces the size of the device. A multiple anode stack consists of a number of units consisting of a cathode, a paper spacer, two or more anodes, a paper spacer and a cathode, with neighboring units sharing the cathode between them. However, to charge and discharge the inner anodes (furthest from the cathode) charge must flow through the outer anodes. With typical anode foil, the path through an anode is quite tortuous and results in a high equivalent series resistance (ESR) for a multi-anode configuration. Thus, ESR increases as more anodes are placed together in the stack. To combat this problem, very low resistivity electrolytes may be used in a multi-anode configuration without an excessive ESR increase.
Another strategy for decreasing the size of the device, is to increase the operating voltage of a single capacitor. If the operating voltage is increased sufficiently, an ICD could be designed with only one electrolytic capacitor. The ability to use a single electrolytic capacitor in an ICD would increase the packaging efficiency, thus reducing the device size. The capacitor, however, would require operating voltages well over 400 Volts. What is needed, therefore, is a capacitor with very high operating voltages and components capable of reliably functioning in a very high voltage electrolytic capacitor.